Government and university scientists say
that even though ozone depleting
substances are beginning to decrease in the atmosphere, it will
be some time yet before scientists are able to observe whether
the ozone layer itself is getting back to normal.

A new study published in the September
16 issue of the Journal
of Geophysical Research estimates that it will be several
decades before an increase in the amount of total overhead ozone
(called "total column ozone") will be detectable.

Elizabeth C. Weatherhead, a University
of Colorado scientist at NOAA's
Air Resources Laboratory in
Boulder, Colo., and colleagues, analyzed predictions from NASA/Goddard's
two dimensional chemical model, along with predictions from nine
other chemical models used in the World
Meteorological Organization's1998
ozone assessment, to estimate the time required to detect
predicted trends of ozone recovery in different areas of the
world. Results indicate that recovery of total column ozone is
likely to show up earliest in the Southern Hemisphere near New
Zealand, southern Africa and southern South America.

"We should expect to be able to detect
recovery in most regions of the world within the next 15 to 45
years," Weatherhead said. "That's based on full compliance
with the Montreal
Protocol and its amendments and no other complicating factors
such as major volcanic eruptions or enhanced stratospheric cooling,"
she continued.

Although there are other ways to detect
signs of ozone recovery, for example, recovery of the ozone layer
at a particular altitude above the earth, total column ozone
measurements provide a complete picture of how much ozone is
present over a region. The total column ozone amount represents
the number of ozone molecules in an imaginary tube 1 centimeter
on a side, stretching upward from the surface to the top of the
atmosphere. Most of this ozone is located high in the atmosphere,
between 20 and 30 kilometers (12 to 18 miles) in altitude. The
ozone in this layer absorbs ultraviolet radiation from the sun,
preventing a portion of the radiation from reaching the earth's
surface where it can be harmful to plants and animals.

Weatherhead says it is crucial to detect
not just a decrease in ozone depleting substances but a recovery
in total column ozone amounts. "Statistical detection of
ozone layer recovery will be an important step toward verification
that all relevant processes in ozone destruction have been identified
and that appropriate measures have been taken to assure the ozone
layer's health. We can't really expect much of an improvement
in the UV levels reaching the biosphere until we can detect an
increase in total column ozone."